Poly(hexamethyleneadipamide), commonly known as nylon 6,6, is used in various forms to manufacture a wide variety of useful products. In particular, nylon 6,6 can be spun into fibers for use in many different applications, including carpets, rugs, upholstery, apparel, sewing-thread, and heavy-duty industrial fabrics, to name but a few. There are, however, several disadvantages associated with making and using nylon 6,6 polymer and fibers, particularly as compared to the other common polyamide, poly(.epsilon.-caproamide) or nylon 6. Among these are the lower dye rate of nylon 6,6 and its tendency to develop spherulites which make both the polymer and the fiber cloudy rather than clear. In addition, nylon 6,6 is thermally unstable at normal processing temperatures. When kept in the molten state at processing temperatures for a long time, the polymer forms branched or crosslinked, insoluble, nonfusible gel. The formation of gel in the polymer product and on the walls of processing equipment can result in deficient product with poor processability, particularly when fiber spinning is involved. The fiber spinning breaks which may occur due to gel formation are troublesome and make the winding of large packages impractical.
Various approaches to overcoming each of these disadvantages have been identified. U.S. Pat. No. 4,559,196, for example, provides for room-temperature dyeability by steam-setting a random copolymer of nylon 6,6 and nylon 6 wherein the nylon 6 constitutes 6-12% of the copolymer. U.S. Pat. No. 4,919,874 discloses a nylon 6,6/6 copolymer fiber having reduced spherulites made by polymerizing nylon 6,6 salt with 1-6% -caprolactam and up to 2% of an inorganic calcium salt. U.S. Pat. No. 3,872,055 reports polyamide compositions having decreased gel-formation wherein the polyamides are copolyamides of the diamine-dicarboxylic acid type containing phosphinic and phosphonous acid compounds together with an alkali metal compound.
Modified nylon 6,6 polymers having a combination of properties which minimize or eliminate such disadvantages would be highly desirable.